Jenny Been

Crack Growth Modeling and Life Prediction of Pipeline Steels Exposed to Near-Neutral pH Environments: Stage II Crack Growth and Overall Life Prediction

This investigation was initiated to provide governing equations for crack initiation, crack growth, and service life prediction of pipeline steels in near-neutral pH (NNpH) environments. This investigation develops a predictive model considering loading interactions occurring during oil and gas pipeline operation with underload-type variable pressure fluctuations. This method has predicted lifetimes comparable to the actual service lives found in the field. This is in sharp contrast with the predictions made by existing methods that are either conservative or inconsistent with the field observations. It has been demonstrated that large slash loads (R-ratio is 0.05), often seen during gas pipeline operation, are a major life-limiting factor and should be avoided where possible. Oil pipelines have shorter lifetime because of their more frequent pressure fluctuations and larger amplitude load cycles. The accuracy of prediction can be improved if pressure data with appropriate sampling intervals are used. The sampling interval error is much larger in the prediction of oil pipelines than gas pipelines because of their different compressibility but is minimized if the pressure sampling rate for the data is at or less than one minute.

Statistical Analysis on Underload-Type Pipeline Spectra

AbstractIn this investigation, analyses of pressure fluctuations during oil and gas pipeline operations are performed. The analyses are performed in a way to capture all the variables of pressure fluctuations and their magnitudes in terms of crack growth rate. It is found that pipeline spectra can be categorized into three main types: underload-dominant, mean load-dominant, and overload-dominant spectra, depending on their locations with respect to a compressor or pump station. The underload spectra—typical of pressure fluctuations at the discharging sites—are the most severe in terms of crack growth as these spectra are subjected to the highest pressure level and the largest magnitude and frequency of pressure fluctuations. The underload spectra are further analyzed in terms of loading and unloading frequency, maximum stress-intensity factor, stress-intensity factor range, numbers of minor cycles between two adjacent underloads, and their potential for crack growth. Special attention is paid to the diffe...

Correlating Corrosion Field Data With Experimental Findings for the Development of Pipeline Mitigation Strategies

Coating disbondment on pipelines is a common phenomenon that leads to exposure of the pipeline metal to ground water solutions, promoting a corrosive environment which is associated with stress corrosion cracking (SCC). This investigation aims to understand the corrosion behavior and rate of pipeline steel under coating disbondments of varying sizes based on field data and experimental studies. In the analysis of the field data, dig reports provided by a Canadian gas transportation company were analyzed for cases of anaerobic corrosion under tape coatings. The analyzed field data provided a correlation between the tape coating disbondment size and corrosion rate found under the coating. The experimental studies aimed to understand the field findings. The analyses were performed on X-65 pipeline steel coupons placed in a vertical coupon holder with a PMMA shielding. To imitate the variation in the disbondment size, the gap size between the metal coupons and the shielding was varied (2 mm, 5 mm, and 10 mm, and infinite). The general corrosion rates were obtained through weight loss calculations. The experimental results were compared and correlated with dig-report data from the field for a development of cathodic protection and pipeline mitigation strategies.Copyright

Surface Crack Growth Behavior of Pipeline Steel Under Disbonded Coating at Free Corrosion Potential in Near-Neutral pH Soil Environments

Crack growth behavior of X65 pipeline steel at free corrosion potential in near-neutral pH soil environment under a CO2 concentration gradient inside a disbonded coating was studied. Growth rates were found to be highest at the open mouth of the simulated disbondment where CO2 concentrations, hence local hydrogen concentration in the local environment, was highest. Careful analysis of growth rate data using a corrosion-fatigue model of the form ΔKα/Kmaxβ/fγ, where (1/fγ) models environmental contribution to growth, revealed that environmental contribution could vary by up to a factor of three. Such intense environmental contribution at the open mouth kept the crack tip atomically sharp despite the simultaneous occurrence of low-temperature creep and crack tip dissolution, which are the factors that blunt the crack tip. At other locations where environmental enhancement was lower, significant crack tip blunting attributed to both low-temperature creep and crack tip dissolution was observed. These factors both led to lower crack growth rates away from the open mouth.

Effect of Pressure Sampling Methods on Pipeline Integrity Analysis

AbstractA computing program has recently been developed to predict corrosion fatigue crack growth in pipeline steel in near-neutral pH environments. Supervisory control and data acquisition (SCADA)...

Depressurization-Induced Crack Growth Enhancement for Pipeline Steels Exposed to Near-Neutral pH Environments

Pressure fluctuations are recognized as the driving force for the crack growth of pipeline steels in near-neutral pH environments; however, the crack growth mechanisms are still not fully understood. Difficulty in understanding the crack growth mechanisms is present due to two dilemmas between laboratory testing and field findings: high frequency study in the laboratory versus low frequency pressure fluctuations in the field; constant amplitude cyclic laboratory tests versus random pressure fluctuations in the actual spectra. To bridge the dilemmas, the crack growth behavior of X60 pipeline steel was investigated in near-neutral pH solution at frequencies as low as 1×10−5 Hz under variable amplitude cyclic loading. Special attention was given to the loading scheme consisting of minor cycles with R ratios (minimum stress/maximum stress) as high as 0.9 and underloads with a relatively lower R ratio of 0.5. It was found that the constant amplitude crack growth rate in near-neutral pH solution in the frequency region below 1×10−3 Hz decreases with decreasing loading frequency, and it reaches a constant value at very low frequencies. This crack growth rate-frequency relation is opposite of that found in the high loading-frequency regime, where crack growth rate was found to increase with decreasing loading frequency. Crack growth rate was observed to increase by a factor of up to 10 when the underload plus minor cycle loading scheme, as mentioned previously, was applied. Based on the findings obtained from the investigation, recommendations of pressure control were also made to minimize the crack growth during pipeline operation.Copyright

Developing prepitting procedure for near neutral pH stress corrosion crack initiation studies on X-52 pipeline steel

Abstract In this paper, a procedure for prepitting of pipeline X-52 steel in the bicarbonate/carbonate solution was developed. Sodium chloride was added to the solution to induce pitting, and the optimum concentration of chloride ions, which produced a low density of pits, was found. The procedure briefly included prepassivation of the steel in the solution with optimum concentration of chloride ions and then increasing the potential above the pitting potential to generate pits. The potential was then stepped back well below the pitting potential to grow the pits to the desirable depth. This procedure can be used to generate pits on tensile specimens, which can be subsequently employed in near neutral pH stress corrosion crack initiation studies. This can result in shorter duration of crack initiation experiments under real loading condition.

Laboratory and Field Investigations of the Performance of HPCC Coatings

Three-layer FBE-polyolefin coatings offer the promise of good adhesive and corrosion properties from the FBE layer coupled with resistance to mechanical damage from the outer polyolefin layer. TransCanada Pipelines have been investigating the long-term behaviour of High Performance Composite Coating (HPCC) using a combination of laboratory testing and field trials. In the laboratory, panels of HPCC were subjected to standard CD disbondment testing following a two-stage degradation process. The degradation process, designed to simulate field exposure, involved impact damage followed by exposure to either a hot-water soak (60°C), or to microbiologically active soil with and without the application of CP. Following exposure, the duplicate panels were subject to 28-day CD disbondment tests to determine the extent of damage caused by the combination of impact and soil/hot water exposure. In the field, a section of HPCC coating was excavated and examined after 11 years service. In addition to visual inspection, the coating was examined in situ using a newly developed impedance technique EISPlus. This technique is a development of earlier EIS techniques and allows the dielectric properties of the coating to be determined in addition to the impedance of the solution-filled pores. EISPlus provides an improved sensitivity for high-impedance coatings, such as FBE, HPCC, and polyolefin tape. Furthermore, since it is a dry technique, rapid measurements can be made on coatings exposed to field conditions allowing the in-service performance to be determined. Results of both the laboratory testing and field EISPlus measurements are presented and the long-term performance of the coating discussed.Copyright

Predicting Near-Neutral pH SCC Conditions Under a Disbonded Coating on Pipelines

Cathodic protection (CP) shielding and the presence of carbon dioxide (CO2 ) in the soil environment are necessary for the occurrence of near-neutral pH stress corrosion cracking (NNPHSCC). Quantitative understanding of the relationship between external conditions, coating deterioration and NNPHSCC is emerging but needs further improvements in modeling and experimental tool. This paper is aimed at understanding the environments initiating the NNPHSCC. New experimental results are presented on crevice chemistry due to the degradation of the mastic of a commercial high-density polyethylene (HDPE) coating and due to CO2 penetration into a disbonded crevice through the coating. Also presented are the results obtained from a comprehensive model, TECTRAN, on the effect of CO2 penetration into a crevice through the holiday alone and through both the holiday and the coating. The experimental results show that as the coating mastic degrades in the soil solution, the solution pH decreases within a few days from about 9 to a steady-state value of about 7.5. The Co2 diffusion through a 0.3 mm commercial HDPE coating is rapid, with a decrease of the soil solution pH from 9 to 5 within a matter of days (external CO2 pressure is 1 atm). The model results show that the presence of CO2 in the soil (0.05 atm partial pressure) can reduce the crevice solution pH to near neutral due mainly to its penetration through the coating, confirming previous hypotheses regarding its role in initiating NNPHSCC.Copyright

Factors Affecting the Generation of High-pH Environments Required for Stress Corrosion Cracking (SCC)

A series of seven experiments have been performed to investigate the effects of various environmental and operational parameters on the generation of high-pH SCC environments under disbonded permeable coating. The parameters investigated include:• the rate of CO2 generation;• direct CO2 production in the disbonded region under the coating;• preferential gas-phase transport of CO2;• the magnitude of the cathodic current density; and• simulated trapped water composition.In general, the total carbonate/bicarbonate concentrations of the simulated trapped water increased with:• increased CO2 flow rate;• direct production of CO2 under the disbonded coating;• increased cathodic current density; and• rapid gas-phase transport of CO2 through holidays above the level of the solution.Ca2+ and Mg2+ cations in groundwaters could buffer the solution in the disbondment at a range of pH 8 to 9 lower than the pH required for the occurrence of high pH SCC. Formation of insoluble carbonate minerals was detected in the simulated disbondment and in the coating with solutions rich in Ca2+ and Mg2+. However, the presence of poorly soluble Ca2+ and Mg2+ cations did not affect the total carbonate/bicarbonate concentration in the inner cell.These observations suggest that environments conducive to high-pH SCC are more likely to occur under the following circumstances:• in regions where the CO2 generation rate is higher, typically wetter and warmer locations;• for coating systems or in regions where there can be microbial activity under the disbonded coating;• in areas where the pipe is not permanently below the water table and where the coating has defects or holidays; and• in areas with a history of higher cathodic current density (which generally results in higher carbonate/bicarbonate concentration) and periodic loss of cathodic polarization.Copyright